Pirouette above the sea,
dive fast on pleated wing
Sand lance for a meal
Why are we sampling?
Of the seabirds that overwinter in Prince William Sound (PWS), nine species were initially injured by the Exxon Valdez oil spill, including three species that have not yet recovered (marbled murrelet, Kittlitz’s murrelet and pigeon guillemot). Long-term monitoring of seabirds in Prince William Sound during winter is needed to understand how post-spill ecosystem recovery and changing physical and biological factors are affecting seabird abundance and species composition, as well as their distribution and habitat use.
Where are we sampling?
We conduct seabird surveys in association with the PWS Herring Research & Monitoring program in various bays within Prince William Sound, including Zaikof, Whale, Eaglek, and Simpson. These bays also provide survey coverage in four different quadrants of PWS.
How are we sampling?
We conduct surveys between mid-September and March following established U.S. Fish and Wildlife Service marine bird survey protocols. One observer records the number and behavior of birds and marine mammals occurring along a strip transect that is 300 meters wide (150 meters on both sides and ahead of the boat, in distance “bins” of 50 meters). Additionally, we record any noteworthy observations out to 1 kilometer on either side of the boat.
In January 2016, in response to reports of hundreds and thousands of dead common murre carcasses washing up along beaches in western Prince William Sound, we conducted an additional survey from January 6-8, 2016, focused on documenting common murre mortalities as part of a larger collaborative effort involving multiple agencies and organizations. We counted floating common murre carcasses while traveling between ocean sampling locations (pelagic transects) and carcasses washed up along the shoreline (beach transects). The total sampling effort covered approximately 400 kilometers: 378 kilometers of pelagic transects, and 22 kilometers of boat-based beach surveys.
What are we finding?
Our surveys demonstrate that seabird species use Prince William Sound differently throughout the nonbreeding season. Common murres, the most abundant wintering marine birds in Prince William Sound, increase in number over the course of winter, peaking in March. Marbled murrelets are absent in October, a time period that coincides with their molt, increase in numbers during November and December, peak in January, and decline in subsequent months. Both glaucous-winged gulls and mew gulls also are most numerous in midwinter, declining as spring approaches. Black-legged kittiwakes are numerous in fall, but occur in very low numbers or are absent between December and February, returning to the Sound in March.
Based on surveys conducted over eight winters we have identified areas of high marine bird concentrations. Notably, northeast Prince William Sound, Montague Strait, and the southwest passages are areas where humpback whales also concentrate. Similarly, Montague Strait is a known hotspot for killer whales. This suggests that in these areas environmental drivers such as currents and nutrients are creating consistent, favorable foraging conditions for upper trophic level predators such as marine birds and marine mammals.
Beginning in March 2015, wildlife officials began receiving reports of dead murres floating in the water or washed up on beaches along the Gulf of Alaska. The die-off continued through the summer and then spiked in December 2015 and early January 2016 after a period of severe storms and high winds. Examination of carcasses sent to the U.S. Geological Survey National Wildlife Health Center determined starvation to be the cause of death.
Immediately preceding the die-off event, we recorded a dramatic increase in the number of common murres using the southwest passages of Prince William Sound in February 2015, rather than the typical peak in March. The early movement of murres into Prince William Sound may have signaled a change in food availability in the Gulf of Alaska due to unusually warm water temperatures. Sea temperatures in the Gulf of Alaska have increased 0.5-1.5 degrees Celsius since 2013, with temperature anomalies in Prince William Sound 2-4 degrees Celsius warmer than average (refer to Oceanographic Conditions in Prince William Sound). Murre densities in Prince William Sound also were higher than previous surveys in October and November 2015, right before the spike in reported mortalities.
During January 2016 surveys to document common murre mortalities, we counted 392 dead murres: 316 on beaches and 76 while traveling. We also collected 6 carcasses for submission to the U.S. Geological Survey National Wildlife Health Center in Madison, Wisconsin. While we did not observe high densities of dead murres compared to other reports, our results indicate that the spatial extent of the die-off in Prince William Sound was quite large. We observed dead murres on 21% of the 122, 3-kilometer segments of surveyed pelagic transects in open water, narrow passages, and bays. Dead murres also were recorded on 7 of the 16 boat-based beach scans.